Create README.md (#10191)

Author: agr

src/NuGet.Services.Sql/README.md

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+# NuGet.Services.Sql
+
+A library that provides support for using Entra ID apps for generating tokens for authenticating SQL connections.
+Authenticating to Entra ID app is done using certificates, which are automatically refreshed from key vault.
+The tokens provided by Entra ID App are cached and reused as long as `AzureSqlConnectionFactory` instance is kept
+alive and reused for creating SQL connections.
+
+## Caveats
+
+When used in ASP.NET/ASP.NET Core app in an Azure app service attempts to retrieve a token from Entra ID app might
+fail with
+
+> Failed to acquire access token for <DB name>.
+
+With inner exception message:
+
+> The system cannot find the file specified.
+
+thrown by `X509Certificate2` constructor.
+
+This can be worked around by [setting](https://stackoverflow.com/a/62790919) `WEBSITE_LOAD_USER_PROFILE=1` variable for the app service.